#include<bits/stdc++.h>
#define endl '\n'
#define pb push_back
using namespace std;
const long long maxn = 905;
void speed()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
}
long long n, m;
long long s[maxn], e[maxn];
map < long long, long long > mp;
vector < long long > u;
vector < long long > g[maxn];
int dp[maxn][maxn];
int over[maxn][maxn];
int pref[maxn][maxn];
vector < pair < int, int > > day, night;
int main()
{
speed();
cin >> n >> m;
for (long long i = 1; i <= n; ++ i)
{
cin >> s[i] >> e[i];
u.pb(s[i]);
u.pb(e[i]);
}
u.pb(0);
u.pb(m);
sort(u.begin(), u.end());
int cnt = 0, last = -1;
for (auto x: u)
{
if(x != last)
{
cnt ++;
mp[x] = cnt;
}
last = x;
}
int lt = mp[0], rt = mp[m];
for (int i = 1; i <= n; ++ i)
{
s[i] = mp[s[i]];
e[i] = mp[e[i]];
}
for (int i = 0; i <= rt + 10; ++ i)
{
for (int j = 0; j <= rt + 10; ++ j)
{
dp[i][j] = 1e9;
over[i][j] = 1e9;
pref[i][j] = 1e9;
}
}
for (int i = 1; i <= n; ++ i)
{
if(s[i] <= e[i])
{
day.pb({s[i], e[i]});
dp[s[i]][e[i]] = 1;
}
else
{
night.pb({e[i], s[i]});
over[e[i]][s[i]] = 1;
}
}
for (int len = 1; len <= rt; ++ len)
{
for (int i = lt; i <= rt; ++ i)
{
int j = i + len - 1;
// cout << i << " " << j << " - > " << dp[i][j] << endl;
if(j > rt)continue;
if(dp[i][j] == 1e9)continue;
for (auto &[l, r]: day)
{
if(r < i || l > j)continue;
int rangel = min(l, i);
int ranger = max(j, r);
dp[rangel][ranger] = min(dp[rangel][ranger], dp[i][j] + 1);
}
// cout << dp[i][j] << endl;
}
}
for (int i = 1; i <= rt; ++ i)
{
for (int j = rt; j >= 1; -- j)
{
// cout << i << " " << j << " -> " << over[i][j] << endl;
if(over[i][j] == 1e9)continue;
for (auto &[l, r]: night)
{
int rangel = max(l, i);
int ranger = min(j, r);
over[rangel][ranger] = min(over[rangel][ranger], over[i][j] + 1);
}
}
}
for (int len = rt; len >= 1; -- len)
{
for (int i = 1; i <= rt; ++ i)
{
int j = i + len - 1;
if(j > rt)continue;
pref[i][j] = min(min(pref[i][j], pref[i-1][j]), dp[i][j]);
if(j < rt)pref[i][j] = min(pref[i][j], pref[i][j+1]);
//cout << i << " " << j << " pref is " << pref[i][j] << endl;
}
}
int ans = 1e9;
for (int i = 1; i <= rt; ++ i)
{
for (int j = 1; j <= rt; ++ j)
{
//cout << i << " " << j << " anddd " << pref[i][j] << endl;
ans = min(ans, pref[i][j] + over[i][j]);
}
}
if(ans == 1e9)cout << -1 << endl;
else cout << ans << endl;
return 0;
}
